Abstract The purpose of this study is to present the source rock characteristics of Late Cretaceous Harshiyat Formation source rock in the onshore and offshore basins of Yemen (i.e. Mukalla-Sayhut, Sayun–Masila, and Jeza-Qamar). Source rock geochemical characterizations were completed and integrated to perform 1-D basin modeling to back-predict source rock thermal maturation, oil generation and expulsion. The geochemical findings revealed that mudstone intervals within the Harshiyat Formation comprised favorable source rocks, with total organic carbon (TOC) content between 0.50 and 35.10 wt %. The organic matter intervals in the Harshiyat mudstones are mainly Types II/III and III kerogens, with certain amount of Type II and I kerogens based on Rock-Eval HI values, ranging from 40 to 923 mg HC/g TOC. Mixed kerogen types were consistent with vitrinite and sapropelic organic matters provided by terrigenous plant inputs, as observed by visual kerogen examination and the distribution of biomarker fingerprints. The existence of mixed types I, II, and III organic matter suggests that the Harshiyat mudstones could contribute to oil and gas at sufficient thermal maturity levels. The biomarker data of the organic matter in the Harshiyat mudstones show the source rock contains a mix marine phytoplanktonic-bacteria/terrestrial land plant enthronement and were deposited in suboxic-anoxic conditions. Results of chemical and optical maturity indicators led to the conclusion of varying maturity stages from immature to late oil window levels for the mudstones of the Harshiyat Formation. Thermal maturation distributions inferred from the analyzed samples indicate that mudstone intervals in the offshore Mukalla-Sayhut Basin are extremely mature, compatible with the late-mature oil window and may have generated significant amounts of oil. This interpretation was supported by strong geochemical similarity between the organic matter in the mudstones and those of the oils found in the offshore Mukalla-Sayhut Basin and demonstrated by the concentrations of n-alkanes and isoprenoids as well as carbon isotopes. In the offshore Ras-Ghashwah-1X well, burial and thermal models were constructed and used to address the effect of tectonic events in triggering oil generation and expulsion from mature Harshiyat source rock in the offshore area. Kinetic database models for Type II and III kerogen mixtures indicate that the initial conversion of kerogen to oil occurred since the Late Oligocene with transformation ratios (TR) that span between 10% and 50%. Furthermore, the oil was expelled from organic matter intervals in the Harshiyat source rock starting from Late Oligocene and continuing until Late Miocene, with TRs of 50%–72%. From that period up to the present, the retained oil was cracked into wet gas in the gas window with an Easy %Ro greater than 1.30, and with peak TR ratios greater than 72%.

中文翻译：

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也门陆上和海上盆地晚白垩世 Harshiyat 组的有机地球化学和盆地模拟：对有效烃源岩潜力和油气生成的影响

摘要 本研究的目的是展示也门陆上和近海盆地（即Mukalla-Sayhut、Sayun-Masila和Jeza-Qamar）晚白垩世Harshiyat组烃源岩的烃源岩特征。完成并整合烃源岩地球化学特征，进行一维盆地建模，反推烃源岩热成熟、生油和排油。地球化学发现表明，Harshiyat 组内的泥岩层段包含有利的烃源岩，总有机碳 (TOC) 含量在 0.50 至 35.10 重量％之间。Harshiyat 泥岩有机质层段主要为 II/III 型和 III 型干酪根，根据 Rock-Eval HI 值，有一定数量的 II 型和 I 型干酪根，范围为 40～923 mg HC/g TOC。通过目测干酪根检查和生物标志物指纹的分布观察，混合干酪根类型与陆源植物输入提供的镜质体和腐泥质有机质一致。混合类型 I、II 和 III 有机质的存在表明 Harshiyat 泥岩可以在足够的热成熟度水平下为石油和天然气做出贡献。Harshiyat 泥岩中有机质的生物标志物数据显示，烃源岩包含混合海洋浮游植物-细菌/陆生植物坐位，并在缺氧-缺氧条件下沉积。化学和光学成熟度指标的结果得出了 Harshiyat 组泥岩从未成熟到晚期油窗水平的不同成熟阶段的结论。从分析样品中推断出的热成熟度分布表明，近海穆卡拉-赛胡特盆地的泥岩层段非常成熟，与晚熟油窗兼容，可能已经产生了大量石油。这种解释得到了泥岩中的有机质与近海穆卡拉-赛胡特盆地中发现的油的强烈地球化学相似性的支持，并由正烷烃和类异戊二烯以及碳同位素的浓度证明。在海上 Ras-Ghashwah-1X 井中，构建了埋藏模型和热模型，并用于解决构造事件在触发海上区域成熟 Harshiyat 烃源岩生油和排油方面的影响。II 型和 III 型干酪根混合物的动力学数据库模型表明，自晚渐新世以来，干酪根最初转化为石油，转化率 (TR) 介于 10% 和 50% 之间。此外，从晚渐新世开始一直持续到晚中新世，Harshiyat 烃源岩有机质层段中的油被排出，TRs 为 50%~72%。从那时到现在，滞留油在气窗中裂解为湿气，Easy %Ro 大于 1.30，峰值 TR 比大于 72%。